One well-known technique for controlling an engine mounted in a vehicle is a torque-based (torque-demand) control which controls the amount of intake air, the amount of injecting fuel, the ignition timing, and the like, based on the torque requested to the engine. A torque-based control calculates a target engine torque, based on the accelerator opening, the engine speed, and the like, for example, and controls the engine such that the target torque is produced. In addition, in a vehicle having external control systems installed therein, such as an automatic transmission, an automatic cruise controller, and a vehicle stabilizer, respective output requests from such external control systems to the engine are centralized and converted into torques, which are then combined into a single value in an engine electronic control apparatus (engine ECU). Thereby the torque behavior of the engine is controlled in a unified comprehensive manner in the engine electronic control apparatus.
In the meantime, the magnitude of the torque produced in the engine depends on the amount of intake air introduced to the cylinder, under the constant combustion conditions, such as the same amount of injecting fuel and the same ignition timing. The amount of intake air varies dependent on the opening of a throttle valve provided on an intake path to the engine. Accordingly, the magnitude of the engine torque can be controlled by adjusting the throttle valve opening.
For example, Patent Literature 1 (Japanese Laid-open Patent Application No. 2009-024677) describes a technique wherein a target torque is converted into a target air amount filled in a cylinder, and a target throttle opening is calculated such that an estimated air amount filled in the cylinder follows the target air amount filled in the cylinder. In this technique, the estimated air amount filled in the cylinder is calculated with a physical model obtained in consideration of the change in the amount of air flowing through the throttle valve and the delay in the intake system, and the throttle opening is controlled such that the estimated amount approaches the target amount.
The flow rate of the air flowing through the throttle valve is represented as a product of the opening area of the throttle valve and the flow velocity of the air. Accordingly, once the flow velocity of the air flowing through the throttle valve is determined, the throttle opening for producing a requested engine torque can be precisely obtained.
For example, Patent Literature 2 (Japanese Laid-open Patent Application No. 2006-132498) describes a technique to calculate a value proportional to the flow velocity of the air (flow rate coefficient), using a ratio (pressure ratio) of the downstream pressure to the upstream pressure of the throttle valve section. In this technique, the pressure ratio is calculated based on signals output from respective pressure sensors provided upstream and downstream to the throttle valve, and an estimate of the amount of air to be introduced to the cylinder is calculated by use of this ratio.
However, in order to calculate the pressure ratio, respective pressure sensors are required for both the upstream and downstream sides of the throttle valve section, and the pressure ratio cannot be calculated if either of the pressure sensors fails. More specifically, the amount of air to be introduced to the cylinder upon adjusting the throttle valve opening is calculated under an assumption that the two pressure sensors are functioning well and are accurate. Therefore, if either of the pressure sensors fails or their detection precision deteriorates, the precision of the estimated air amount may be deteriorated and the controllability of the engine torque based on this amount of intake air may be compromised and degraded.
As a fail safe in case of a failure of a pressure sensor, it is considered to provide a technique for estimating the amount of air to be introduced to the cylinder without using a pressure ratio. For example, the relations between throttle valve openings and the amounts of air to be introduced to the cylinder with these openings are defined in a map or table, which is stored in the engine ECU. However, a map or table corresponding to a wide variety of driving conditions should be defined beforehand, which consumes larger ROM space in the engine ECU and boosts the development cost.